1. Field of the Invention
The present invention generally relates to an objective lens driving apparatus of an optical disk drive and, more particularly, to an objective lens driving apparatus having an actuator for moving an objective lens in a focusing direction and a tracking direction.
2. Description of the Related Art
FIGS. 1A and 1B show a first example of a conventional objective lens driving apparatus. In the objective lens driving apparatus shown in FIGS. 1A and 1B, an objective lens 1 is supported by an objective lens supporting member 2 that is elastically supported by four wire springs 4 extending from a stationary member 3. A focusing coil 5 and a tracking coil 6, which are wound in directions perpendicular to each other, are provided around the objective lens supporting member 2. An end of each of the wire springs 4 is soldered to a base board 7 at a soldered portion 8a. The opposite end of each of the wire springs 4 is soldered to a respective one of the focusing coil 5 and the tracking coil 6 at a soldered portion 8b. Accordingly, the wire springs 4 serve as conductive wires to provide an electrical connection to the focusing coil 5 and the tracking coil 6. Magnets 10 are fixed to yokes 9 so that the objective lens supporting member 2 is interposed between the magnets 10. A light beam having an optical axis indicated by a chain line 12 is deflected by a prism 11, and enters the objective lens 1.
In the above-mentioned structure, the objective lens 1 is moved in a focusing direction indicated by an arrow Z and a tracking direction indicated by an arrow Y by moving the objective lens supporting member 2 by providing control currents to the focusing coil 5 and the tracking coil 6.
A description will now be given, with reference to FIG. 2, of a second example of the conventional objective lens driving apparatus. FIG. 2 is a perspective view of the second example of the conventional objective lens driving apparatus. In FIG. 2, parts that are the same as the parts shown in FIGS. 1A and 1B are given the same reference numerals, and descriptions thereof will be omitted.
In the objective lens driving apparatus shown in FIG. 2, each of the two magnets 10 is fixed to a respective one of yokes 9a. The focusing coil 5 and the tracking coil 6 are situated around the objective lens supporting member 2. An additional yoke 9b is provided near a respective one of the yokes 9a so that the focusing coil 5 and the tracking coil 6 are interposed between the yokes 9a and 9b. Accordingly, a closed magnetic circuit is formed by each of the magnets 10 and a respective pair of the yokes 9a and 9b. Thus, similar to the first example, the objective lens 1 is movable in a focusing direction indicated by an arrow Z and a tracking direction indicated by an arrow Y by moving the objective lens supporting member 2 by providing control currents to the focusing coil 5 and the tracking coil 6.
In the above-mentioned first and second examples of the conventional objective lens driving apparatus, the prism 11 must be situated under the objective lens supporting member 2 as shown in FIG. 1B since the focusing coil 5 and the tracking coil 6 are present. Thus, there is a problem in that a thickness of the entire apparatus is increased due to the arrangement of the prism 11.
Additionally, Japanese Laid-Open Patent Application No.9-180207 discloses a third example of the conventional objective lens driving apparatus. In this objective lens driving apparatus, a deflection mirror is provided inside the objective lens supporting member 2 so that the deflection mirror is movable together with the objective lens 1. Thereby, a light beam is prevented from being offset when the objective lens supporting member 2 is moved in the tracking direction.
It is considered to adopt a structure shown in FIG. 3 in which structure a driving motor having a closed magnetic circuit similar to the above-mentioned second example is applied to the structure disclosed in Japanese Laid-Open Patent Application No.9-180207. However, in such a case, there is a problem in that the objective lens supporting member 2 is tilted due to a torque generated by an offset between a center of driving force exerted on the focusing coil 5 and the center of gravity of a movable part including the focusing coil 5 as shown in FIG. 4.
Additionally, since the tracking force is generated by the tracking coil 6 in which a current flows in the focusing direction, there is a problem in that a sufficient drive force cannot be obtained.
Further, the above-mentioned Japanese Laid-Open Patent Application No.9-180207 discloses another embodiment in which an actuator for tracking is formed by a magnetic circuit different from a magnetic circuit of an actuator for focusing. Such a structure has a problem in that an assembling operation is complex and a manufacturing cost is increased.
It is a general object of the present invention to provide an improved and useful objective lens driving apparatus in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an objective lens driving apparatus in which a focusing coil and a tracking coil can be arranged in a parallel relationship with a thin, flat shape so that both the focusing coil and the tracking coil can be moved by a single flat magnet.
Another object of the present invention is to provide an objective lens driving apparatus in which a movable part including a focusing coil is prevented from being tilted.
A further object of the present invention is to provide an objective lens driving apparatus in which drive forces generated by a focusing coil and a tracking coil are increased by forming a closed magnetic circuit.
In order to achieve the above-mentioned objects, there is provided according to the present invention an objective lens driving apparatus for moving an objective lens in a focussing direction and a tracking direction relative to an optical disk, the objective lens driving apparatus comprising:
an objective lens supporting member supporting the objective lens, the objective lens supporting member being movably supported on a stationary member; and
a drive motor provided at least one side of the objective lens supporting member, the drive motor comprising:
a yoke fixed to the stationary member;
a drive magnet mounted to the yoke and having a flat surface parallel to both the focusing direction and the tracking direction, the flat surface having four single-pole areas each of which is magnetized in a direction perpendicular to the flat surface; and
a drive coil assembly mounted on the objective lens supporting member and including a focusing coil and a tracking coil, the drive coil assembly having a flat shape so that the drive coil assembly faces the flat surface of the drive magnet in parallel relationship with a small air gap therebetween.
According to the above-mentioned invention, the drive motor is formed by the combination of the drive magnet having a surface divided into four magnetization areas and the drive coil assembly having a flat shape. The drive magnet generates magnetic fields in different directions in the vicinity of the flat surface. Accordingly, both the focusing coil and the tracking coil can be positioned in the vicinity of the drive magnet and parallel to the flat surface so that both the focusing coil and the tracking coil generate a drive force by being subjected to the magnetic fields in the vicinity of the drive magnet. Thus, the drive motor can be formed in a flat shape. Since the focusing coil does not surround the objective lens supporting member, a light beam can be introduced into an optical system provided within the objective lens supporting member in the tracking direction. Additionally, the entire focusing coil can be subjected to the magnetic field generated by the drive magnet, a focusing force is generated uniformly in the entire focusing coil. Thereby, it can be assumed that the focusing force is exerted on the center of gravity of the focusing coil. Thus, the objective lens supporting member to which the focusing coil is mounted is prevented from being tilted due to an offset between the center of gravity of the focusing coil and the center of the area which receives the focusing force.
In the objective lens driving apparatus according to the present invention, the drive motor may further comprise an additional yoke fixed to the stationary member and an additional drive magnet mounted to the additional yoke, the additional drive magnet having the same magnetization structure as the drive magnet, the additional drive magnet being positioned adjacent to the drive magnet so that the drive coil assembly is interposed therebetween and a closed magnetic circuit is formed by a combination of the yoke, the drive magnet, the additional drive magnet and the additional yoke.
According to this invention, the drive coil assembly is interposed between the two drive magnets that are mounted on the respective yokes. That is, the drive coil assembly is positioned in a closed magnetic circuit. In such a closed magnetic circuit, a high, uniform magnetic flux density can be obtained. Thus, the drive motor can effectively generate both a focusing force and a tracking force.
According to one embodiment of the present invention, the single-pole areas of the drive magnet are defined by a first line extending in a focusing direction and a second line perpendicular to the first line and extending in the tracking direction, each of the single-pole areas is magnetized in a direction opposite to a direction of magnetization of adjacent ones of the single-pole magnets;
the focusing coil including a first focusing coil and a second focusing coil, the first focusing coil being positioned on one side of the first line and extending over the second line, the second focusing coil being positioned on the other side of the first line and extending over the second line; and
the tracking coil including a first tracking coil and a second tracking coil, the first tracking coil being positioned on one side of the second line and extending over the first line, the second tracking coil being positioned on the other side of the second line and extending over the first line.
Additionally, according to another embodiment of the present invention, the single-pole areas of the drive magnet includes a first single-pole area, a second single-pole area, a third single pole area and a fourth single pole area defined by a first line extending in the focusing direction and a second line extending in the tracking direction, the first and second single-pole areas being positioned on one side of the first line and the third and fourth single-pole areas being positioned on the other side of the first line, the first and fourth single-pole areas being positioned on one side of the second line and the second and third single-pole areas being positioned on the other side of the second line, each of the single-pole areas being magnetized in a direction opposite to a direction of magnetization of adjacent ones of the single-pole areas;
the focusing coil includes a first focusing coil and a second focusing coil, the first focusing coil being positioned on one side of the first line and extending over the second line so that a half portion overlaps the first single-pole area and the other half portion overlaps the second single-pole area, the second focusing coil being positioned on the other side of the first line and extending over the second line so that a half portion overlaps the third single-pole area and the other half portion overlaps the fourth single-pole area; and
the tracking coil includes a first tracking coil, a second tracking coil and four third tracking coils, the first tracking coil being positioned on one side of the second line and extending over the first line so that a half portion overlaps the first single-pole area and the other half portion overlaps the fourth single-pole area, the second tracking coil being positioned on the other side of the second line and extending over the first line so that a half portion overlaps the second single-pole area and the other half portion overlaps the third single-pole area, a first one of the third tracking coils being positioned on one side of the first tracking coil so that at most a half portion of the first one of the third tracking coils overlaps the first single-pole area, a fourth one of the third tracking coils being positioned on the other side of the first tracking coil so that at most a half portion of the second one of the third tracking coils overlaps the fourth single-pole area, a second one of the third tracking coils being positioned on one side of the second tracking coil so that at most a half portion of the second one of the third tracking coils overlaps the second single-pole area, a third one of the third tracking coils being positioned on the other side of the second tracking coil so that at most a half portion of the third one of the third tracking coils overlaps the third single-pole area.
Further, according to one embodiment of the present invention, the single-pole areas of the drive magnet includes a first single-pole area, a second single-pole area, a third single pole area and a fourth single pole area that are defined by a first line extending in the focusing direction, a second line parallel to the first line and a third line extending in the tracking direction between the first line and the second line, the first single-pole area being positioned on one side of the third line and between the first line and the second line, the second single-pole area being positioned on the other side of the third line between the first line and the second line, the third single-pole area being positioned on a side of the first line which side is opposite to a side on which the first and second single-pole areas are positioned, the fourth single-pole area being positioned on a side of the second line which side is opposite to a side on which the first and second single-pole areas are positioned, the first single-pole area being magnetized in a direction opposite to a direction of magnetization of the second single-pole area;
the focusing coil is positioned between the first line and the second line and extends over the third line so that a half portion overlaps the first single-pole area and the other half portion overlaps the second single-pole area; and
the tracking coil includes a first tracking coil and a second tracking coil, the first tracking coil being positioned on one side of the focusing coil so that at most a half portion overlaps the third single-pole area, the second tracking coil being positioned on the other side of the focusing coil so that at most a half portion overlaps the fourth single-pole area.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanied drawings.